The invention generally relates to improvements of the thermal behavior of an optical component. More specifically, the invention relates to improvements of the thermal behavior of a collector for use in EUV-lithography.
Although the present invention will be particularly described with respect to a collector for use in EUV-lithography, the present invention is not limited thereto.
It is to be understood that, in general, the invention is also useful for other optical components.
Simulation results for the collector of an illuminating system in the extreme ultraviolet (EUV) range show that the collector strongly heats up in operation. As a consequence of the temperature rise, the collector deforms.
With reference to FIG. 1 of the accompanying drawings, the heating of a general optical component will be briefly explained. FIG. 1 shows an incident optical beam 1 impinging on an optical component. The optical component is, for example, a mirror, comprising a substrate body 4 and an optical layer 3 as the optical element, at which the beam is deviated or reflected as shown by reference numeral 2. Other optical components like lenses, prisms, gratings, beam splitters, etc., are conceivable.
A portion of the impinging energy 1 is absorbed by the optical layer 3 or by the substrate 4. Heat 5 is generated thereby which spreads in the substrate 4. In most cases, this heat is poorly carried off, because the fixing element 6 to the holder 7 and also the substrate material 4 have a poor heat conduction ability. The substrate 4 expands, while this expansion is hindered by the fixing elements 6 and the holder 7, which leads to local deformations of the optical element (substrate and/or optical layer) and, thus, to a deterioration of the optical performance of the device, in which the optical component is used.
Depending on the power of the illumination of the optical system, certain temperatures of the system arise. If no heat carrying off is provided by heat conduction into a cooling medium, the temperatures can become pretty high and rapidly exceed an operating temperature which is in some cases required.
EP 1 387 054 A2 discloses a cooling apparatus and a cooling method for cooling an optical element provided in a vacuum atmosphere. The cooling apparatus includes a radiational cooling part, arranged apart from the optical component for cooling the optical component by radiation heat transfer, and a controller for controlling the temperature of the radiation cooling part.
This cooling apparatus is expensive, because it requires a considerable number of additional parts, thus also increasing the space requirements of the optical component.
US 2004/0051984 A1 discloses devices and methods for cooling devices, optical elements and optical systems, including optical systems used in vacuum environments. The cooling device comprises a heat-receiving plate arranged proximally to a respective optical component along a surface of the optical component at which light directed to the optical component is not incident or outgoing. The heat-receiving plate is configured to receive heat from the optical component. Again, this known device requires a number of parts in addition to the parts of the optical component for cooling same.
The technical problems occurring as a result of the heating of the optical element can be summarized as follows. On the one hand, the optical component becomes too hot, as a result of which the substrate material and the optical layers can be destroyed. On the other hand, the optical component can be deformed so severely that the optical performance of the system does not conform to the required specification. Finally, the deformation of the optical component can change during operation, that is represent a transient effect. A one-off or static correction of the resultant error in the optical system, for example with the aid of other optical components, would therefore be inadequate.